The invention pertains to a process for the catalytic gas phase dehydrogenation of hydrocarbons, particularly of alkyl aromatics.
The catalytic gas phase dehydrogenation of hydrocarbons, such as alkyl aromatics, especially of ethylbenzene, but also of saturated or mono-unsaturated aliphatic hydrocarbons such as propane or butene, in the presence of water vapor at an increased temperature (in general in the temperature range of 500.degree. to 700.degree. C.), is a process executed on a large scale industrially. Depending on the type of heat supply, there is an isothermal and an adiabatic form of embodiment of the process. A thorough description of the invention processes for the dehydrogenation of alkyl aromatics is found in "Industrielle Aromatenchemie" ["Industrial Aromatics Chemistry"] by H. G. Frank and J. W. Stadelhofer, Springer Publishing House, Berlin--Heidelberg 1987, pages 142-147. The catalysts used for these processes are metal oxide catalysts. The most effective catalysts contain iron oxide as the primary component, plus oxide-forming alkali metal salts, as well as structure-stabilizing and activity-and selectivity-increasing metal compounds. These types of catalysts are described in EP-A-0 177 832, for example. The catalysts are usually used in the form of molded cylindrical pellets, rarely as tablets, spheres, or rings. The particle diameter of the molded catalysts particles usually varies between 1.5 and 12 mm. For isothermal processes, the particle size must be at least 5 mm on account of the high pressure loss in the tubes of the dehydrogenation reactor. In the radial reactors which are used in adiabatic processes, catalyst pellets with a diameter of approximately 1.5 to 5 mm and a length/diameter ratio of approximately 0.5 to 3 can generally be used in the shorter fixed bed packings.
For kinetic reasons, as small a catalyst particle size as possible is desired, so that maximal yields of the desired dehydrogenation products are obtained from the selective dehydrogenation reaction with only short retention times in the catalyst particle, i.e. for short diffusion distances of the products and products to the activated catalyst surface area and back into the gas phase.
Small particle dimensions cause a high pressure drop, however. For thermodyamic reasons, as low a pressure as possible is desired for the dehydrogenation reaction.
According to DE-AS 25 44 185 and EP-B-0 206,192, which have as their object processes for the catalytic dehydrogenation of alkyl aromatics, particularly of ethylbenzene, this requirement is taken into account by the choice of special catalyst geometries, e.g., cylinder ring, star or cross pellet, and honeycomb shape.
Although an increased catalytic activity relative to cylindrical pellets is attained with the use of this type of molded catalyst particle, the objective remains to further improve the activity and selectivity of the dehydrogenation catalysts via a kinetically more favorable shape, because of the great industrial significance of processes for the catalytic dehydrogenation of hydrocarbons, particularly for the production of styrene.
From DE-A-31 41 942 (U.S. Pat. No. 4,328,130) molded catalyst particles are known with a cylindrical shape with several axial indentations which extend radially from the cylinder periphery inwards and define the elevations lying between them, whose maximum width is larger than that of the indentations. These molded catalyst particles are used for the conversion of hydrocarbons, particularly in the isomerization, alkylation, reforming and hydrogenation, including hydrocracking, hydrotreating, hydrorefining, hydrometallization, hydrodesulfurization, and hydrodenitrification. These reactions are carried out in the liquid phase, and as such, high retention times for liquids are desired, which are promoted by the clover-shaped cross-section in conjunction with the comparably smaller diameter (&lt;0.23 cm) of the molded catalyst particles.
Moreover, said liquid-phase reactions are preferably carried out with carrier catalysts, with aluminum oxide preferably used as the carrier material, onto which metal, metal oxides and metal sulfides from the transition elements (e.g., nickel, cobalt, molybdenum, and/or tungsten) can be applied as the catalytically active substance. These types of catalysts are not suitable for catalytic gas phase dehydrogenation of hydrocarbons, because they are deactivated in a short period of time due to carbon deposits.